Avalanche!

A look at how avalanches occur
SLAB FORMATION AVALANCHES
These are the most dangerous type of avalanches. They occur when layers of snow of varying density build up and are destabilized by weight. Wind can easily deposit 60 cm of snow in two hours, producing a huge weight very fast.
Starting zone: Surface snow cracks and slides away from fracture line. Avalanche track: Path that snow follows as it goes downhill. Runout zone: Snow and debris stops moving — victims most likely to be buried here. Fresh snowfall: Can increase mass to critical level. Buried surface hoar: Icy crust formerly at surface. Depth hoar snow: Large, low-density crystals formed by rising water vapour. As the temperature rises, the bottom layer becomes very slippery. A slight weight can trigger the top layer of snow to loosen and slide down the mountain.

IN MINUTES

News and events — visually
Two people have recently died in B.C. avalanches, bringing the total deaths to three for this season.

WEATHER-PRODUCED AVALANCHES Wind: Most common cause of weather-produced avalanches. Wind can deposit snow 10 times faster than snow falling from storms. Snowstorms: If the weight of new snow is added faster than the buried weak layer can adjust to its load, then it fractures and forms an avalanche. Rapid warming: Rare. Avalanches caused by rain or melting of surface snow occur because of a decrease in strength of the buried weak layer. The water dissolves the bonds between the snow grains.

Duncan MacKenzie, 35, a long-time ski patroller, was killed in a snowslide east of Pemberton, B.C., on Thursday.

1

Greg Sheardown. 45, was killed while heli-skiing near Revelstoke, B.C.

2

B.C.

1 2
Heli-skiing

Alta. Sask.

In 90% of avalanche accidents, the victim or someone in the victim’s party triggers the avalanche. Sound cannot trigger and avalanche.
Windward side Ridge line Leewar side Danger area

FATALITIES
30 25 20 15 10 5 0 ’70
ﬂoor

Number of avalanche fatalities, winters 1970-2010 Canada

O -trail, downhill skiing accessed by a helicopter, not a ski lift. As of April, 2011, no one heli-skiing or cat-skiing died in avalanches in 2010-11, while two people died this way in the previous ski season. Twenty-six “mechanized skiers,” which includes heli- and cat-skiers, have been killed by avalanches since 1993-1994, with nine losing their lives in 1996-1997.

AVALANCHE DANGER AREAS

Wind direction

Fatalities by type of activity, 2000-2010 10% 8%
Mechanized skiing

Other

5%

The safest areas to travel are along the windward slope of the ridge line. Or along the valley ﬂoor, away from the leeward slope.

30 km/h

Mountaineering
’75 ’80 ’85 ’90 ’95 ’00 ’05 ’10

41%
Backcountry skiing

Speed at which an average-sized dry avalanche travels — it’s nearly impossible for someone to outrun an avalanche.